The present invention relates, in general, to the pulp and paper industry, and in particular, to a new and useful apparatus and method or technique for rapid and accurate measurements of physical and chemical properties of individual wood pulp fibres such as fibre coarseness, width, wall thickness, and lignin content.
To ensure paper quality, it is important to know the physical properties of wood pulp fibres used in papermaking. Important properties include fibre length, and transverse dimensions such as cross-sectional area, width, perimeter, and wall thickness as shown in FIG. 1 [1,2]. While the major effect of fibre length is on the sheet strength, fibre transverse dimensions affect all paper properties structural, strength, and optical. Unfortunately, many important fibre transverse dimensions have been difficult to measure. Moreover, all fibre properties are distributed in nature. The information on the distributions of fibre properties is considered more important than their mean values in controlling pulp quality as it provides the extent of heterogeneity in a pulp, and allows us to identify the amount of fibres with undesirable properties.
Fibre coarseness, defined as mass per unit length and related to the fibre cross-sectional area by the density of fibre wall materials, is an important fibre property [1,2]. Optical instruments, such as the Kajaani fibre length analyser (Kajaani Electronics Ltd, Finland), the Fibre Quality Analyser (Optest, Canada)[P1], and fibre length analyser (Andritz Sprout-Bauer, Inc., US) [P2] were developed for the rapid determination of fibre-length distribution. If the total mass of pulp fibres being measured is known, these instruments will calculate population-average fibre coarseness. This technique can neither provide the information on fibre coarseness distribution, nor be implemented for an on-line measurement of coarseness. A rapid and accurate method for measuring the coarseness of individual wood pulp fibres is not yet available because of their extremely small weight and irregular shape.
Fibre wall thickness is another important fibre property. Two fibres of similar coarseness can have quite different wall thickness if their perimeters are different. Recently, a new instrument, Kajaani FibreLab fibre analyzer, provides measurements for fibre width and cell wall thickness of fibres flowing through a capillary tube [P5]. The principle of this instrument is based on microscopic imaging. This measurement technique is quite adequate for fibre width because its dimension is in the range of tens of microns.
However, this direct imaging technique faces many difficulties for accurate fibre wall thickness measurements. First, an accurate measurement for fibre wall thickness, which is in the range of a few microns, requires high resolution, and therefore, high precision optics and precise focusing. Precise focusing is difficult to accomplish for a flowing fibre. Second, the measurement is based on the projected two-dimensional image of a fibre. The interpretation of image can be complex and difficult. Third, this wall thickness measurement, at best, is obtained only from two sides of the fibre, but not around the whole fibre. Therefore, the measurement depends on the orientation of the fibre, as the wall thickness varies around the fibre. And finally, the direct imaging method can only measure the apparent fibre wall thickness that depends on the degree of fibre wall swelling and delamination, or external fibrillation, but not the true fibre wall thickness. Thus, a rapid and accurate technique for measuring the wall thickness of individual fibres is still lacking.
Recently, a nondestructive procedure has been developed for obtaining cross-sectional images of wood pulp fibres using the optical sectioning ability of confocal laser scanning microscopy (CLSM) [3]. When combined with image analysis, this technique is capable of accurately measuring individual fibre transverse dimensions, such as wall thickness and cross-sectional, hence, fibre coarseness [4]. Although this technique provides much valuable information on fibre quality, and is a good research tool, it is too slow for most practical purposes. A new rapid technique with similar or comparable accuracy as in the CLSM technique for measuring individual fibre transverse dimensions is needed.
In a chemical pulp manufacturing process, the production of wood pulp fibres and/or paper products from wood chips is by removing, either partially or entirely, lignin from the wood. Lignin content is an important quality parameter and property for chemical pulp fibres. The amount of lignin left in a pulp after chemical pulping process is measured in terms of Kappa number. There are a few commercial Kappa Number Analyzers available for measuring the Kappa number in a pulp. However, the importance of uniformity to product quality arises not only from the physical properties of fibres, but also from their chemical properties. Unfortunately, few data are available on lignin content variability within and between individual fibres. Methods to determine the kappa number of individual pulp fibres include use of a density gradient column and Fourier transform infrared (FTIR) microscopic analysis [5], and an intensity measurement of primary fluorescence [P4]. Recently, Liu et al. described a method based on fluorescence microphotometry of fluorescent stained fibres [6]. However, these methods are either too slow or not reliable. There is as yet no rapid and reliable technique or apparatus for measuring the lignin content/Kappa number of individual fibres.
It is known that wood, pulp and paper samples exhibit inherent fluorescence. This fluorescence is the sum of the fluorescence from cellulose, hemicellulose, lignin and the lignin artefacts generated during the pulping process [7]. The fluorescence spectra of mechanical and chemical pulp sheets have been investigated in a number of studies. In general, these studies found similar broadband emission spectra for all pulp sheet samples at a given excitation wavelength. For example, the fluorescence emission spectra obtained using 350 nm excitation light have broad, structureless bands between 375 and 600 nm, and have maxima around 450 nm.
Fluorescence from wood fibres is a very complex process. It is known that fluorescence from paper or pulp is a highly non-linear function of sheet basis weight or grammage and the excitation wavelength. It also shows an unpredictable dependence on lignin content. For example, increasing lignin content can lead to a decrease in fluorescence because of re-absorption mechanism [8]. Thus, it is uncertain whether fluorescence intensity can be used for quantifying physical and chemical properties of wood pulp fibres. Recently, techniques based on optical fluorescence spectroscopy have been used in determining the chemical composition, for example, the local abundance of lignin in paper [8]. Jeffers et al. described a method for on-line measurement of lignin in wood pulp by color shift of fluorescence [P3]. However, these techniques suffer from the problems normally associated with the fluorescence from pulp and paper. For instance, decreasing lignin content is shown to produce an increase in fluorescence intensity. The fluorescence spectra are expected to be affected by the above-mentioned problems.
The mismatch of refractive indexes in fibres and water create optical discontinuities in the fibre wall and water interfaces. Methods based on optical methods for measuring transverse dimension measurements on fibres suspended in water face issues such as interferences from light scattering. Moreover, optical measurements depend on the complex relationship between optical properties, light scattering, and orientations of the fibres being evaluated.
Therefore, there is still a need for a rapid and accurate technique for measuring physical and chemical properties of individual fibres.
It is an object of this invention to provide a method of determining physical and chemical properties of particles, especially wood pulp fibres.
It is another object of this invention to provide an apparatus for determining physical and chemical properties of particles, especially wood pulp fibres.
In one aspect of the invention there is provided a method of determining a physical or chemical parameter of wood pulp comprising: a) applying excitation light at at least one predetermined wavelength to wood pulp, to produce fluorescence emission light from individual fibre particles of the wood pulp, b) detecting fluorescence intensities of said fluorescence emission light, for each said predetermined wavelength, and c) determining a physical or chemical parameter of individual fibre particles of the wood pulp from said fluorescence intensities.
In another aspect of the invention there is provided an apparatus for determining a physical or chemical parameter of wood pulp comprising: i) means to apply excitation light at at least one predetermined wavelength to wood pulp, to produce fluorescence emission light from individual fibre particles of the wood pulp, ii) detection means for detecting fluorescence intensities of the fluorescence emission light for each predetermined wavelength, and iii) means for determining a physical or chemical parameter of individual fibre particles of the wood pulp from the fluorescence intensities.
The invention relies mainly on fluorescence properties of fibre-like particles to provide a method, and apparatus or measurement instruments implementing the method to measure physical and chemical properties of individual fibre-like particles, in particular wood pulp fibres, simultaneously if needed.